Heterogeneous cellular communication networks (e.g. as specified by the Third Generation Partnership Project, Long Term Evolution—3GPP LTE) typically deploy a mixture of cells of differently sized and overlapping coverage areas. The larger cells are typically handled by high power nodes and the smaller cells are typically handled by low power nodes. The high power nodes may, for example, be traditional cellular communication nodes (e.g. macro nodes). Some examples of low power nodes in heterogeneous communication networks include pico nodes, home base stations (femto nodes), and relays. The various types of cells are organized into two or more cell layers of the heterogeneous network. Terminology from 3GPP LTE specifications and implementations will be used throughout this disclosure for illustrative purposes. This is not to be considered limiting. Contrarily, the description herein may be equally applicable to other suitable scenarios, e.g. based on WCDMA (wideband code division multiple access), HSPA (high speed packet access), GSM (global system for mobile communication), EDGE (enhanced data rate for GSM evolution), etc.
A simple example of a heterogeneous network deployment is illustrated in FIGS. 1A-C, where two neighboring macro cells 31, 41 and a pico cell 21 deployed within the coverage area of the macro cell 31 are shown.
One aim of deploying low power nodes, such as pico nodes, within the coverage area of a macro cell is to improve system capacity via cell splitting gain. Another aim is to provide very high speed data access to end users throughout the network or at least in selected areas. Heterogeneous network deployments are particularly effective for providing service in traffic hotspots, i.e. small geographical areas with high user density. In a heterogeneous network deployment, traffic hotspots may be served by, for example, pico cells. Thus, heterogeneous network deployments represent an alternative to a denser macro network deployment.
Because of the, sometimes substantial, difference in output power between high power nodes and low power nodes (e.g. 46 dBm for macro cells and less than 30 dBm for pico cells in an example scenario), the interference situation in heterogeneous networks typically differs from that of a traditional one-layer cellular communication network where all base stations have the same or similar output power.
The communication associated with the various layers of a heterogeneous network need to be separated so that a manageable interference situation may be provided. Such separation may, for example, be provided via frequency separation or via resource coordination.
In the frequency separation approach, nodes that serve cells associated with different layers of the heterogeneous network operate on different and non-overlapping carrier frequencies. Thereby, interference between the layers is avoided. With no higher (e.g. macro) layer cell interference towards the lower (e.g. pico) layer cells, cell splitting gain is achieved when all communication resources are used by the lower layer cells. Generally, when referred to herein, the higher and lower layer cells may be as defined according to any relevant known or future heterogeneous network terminology. For example, the higher layer cell may be a macro cell and the lower layer cell may be a pico or femto cell. Other relevant examples of a higher layer cell include an aggressor cell (e.g. as used in 3GPP specifications) and a source cell. Other relevant examples of a lower layer cell include a victim cell (e.g. as used in 3GPP specifications), a hotspot cell and a target cell One drawback of frequency separation is that it may lead to inefficient resource utilization due to that the split of carrier frequencies between layers is typically done in a static manner. For example, when there are low traffic amounts in the lower layer cells located in the coverage area of a higher layer cell, it might be more efficient to use all available carrier frequencies in the higher layer cell and basically switch off the lower layer cells. Such an approach would, however, not be possible due to the static carrier frequency assignment.
In the resource coordination approach, radio resources of one or more carrier frequencies may be shared between layers by cross-layer coordination of transmissions, wherein some radio resources (e.g. some resource blocks of 3GPP LTE) are allocated to a higher layer cell during a time period while the remaining available radio resources may be accessed by the lower layer cells without interference from the higher layer cell. This type of coordination may be referred to as inter-cell interference coordination (ICIC). Depending on the traffic situation across the layers, the resource split may change dynamically over time to accommodate different traffic demands. Efficient application of ICIC requires time synchronization between base station nodes.
A wireless communication device (sometimes simply referred to as a device herein) typically has to perform a number of measurement and synchronization operations before it can connect to or communicate with a cellular communication network. Using 3GPP LTE as an example, the wireless communication device typically first performs a cell search where it may find and acquire synchronization to one or more cells within the network. Then, information needed to communicate with and operate properly within one of the cells is typically received and decoded, and the cell may be accessed using the so-called random-access procedure. This type of procedures are typically performed at power up of the wireless communication device, but also throughout the operation of the device to support mobility and/or changing radio conditions.
For a decision regarding which cell the wireless communication device should use (e.g. decision by the network to perform handover for terminals in active mode, and decision by the device cell to perform re-selection for terminals in idle mode), an estimation of communication quality of the cells at hand is typically applied. Such a decision may be based on the received power of each of the cells under consideration, on the path loss of each of the cells under consideration, or a combination. A situation where the decision differs depending on which of the above measures is applied is referred to as a link imbalance situation and is quite typical for heterogeneous networks due to the difference in output powers between the nodes of different layers, and ICIC would typically be particularly beneficial in a region where a lower layer cell is used even though a higher layer cell offers a stronger received power.
In a typical communication standard supporting heterogeneous network deployments (e.g. 3GPP LTE), some or all heterogeneous network procedures for a wireless communication device may only be defined when the device is in an active mode. Thus, searching for low power (e.g. target, hotspot, victim, etc.) node signals (which may be drowned by high power (e.g. source, macro, aggressor, etc.) node signals) to find lower layer cells and/or performing measurements according to a restricted measurement pattern may be performed only in active mode. Hence, when a device is in an idle mode it typically cannot camp on a lower layer cell if not the difference in received power between higher layer cells and the lower layer cell is small (e.g. 6 dB in 3GPP LTE).
Thus, a wireless communication device that is served by a low power node in active mode typically changes to camping on a cell provided by a high power node when transferring to idle mode. When entering active mode again, the device typically uses the camping cell as serving cell (i.e. sets up a connection to the high power node). Once in active mode, the device will be subject to heterogeneous network procedures again and will eventually be subject to handover to the lower layer cell of the low power node (if the device is still within reach of the low power node). This cell switching will create some overhead signaling between the device and the network as well as in the network backhaul between lower and higher layers (e.g. over the X2 interface of 3GPP LTE). The problem with overhead signaling will be particularly detrimental in situations with frequent mode changes of devices, which becomes more prominent with the increasing use—for example in smartphones—of various applications that frequently access the network (i.e. to check for content updates or similar).
This may be partly solved by letting the device stay in active mode with discontinuous reception (DRX). Such a solution, however, requires more network and radio resources than the idle mode and is not preferable (except maybe for an initial phase of, for example, between 1 and 60 seconds, before transferring to idle mode).
Therefore, there is a need for methods and arrangements that overcome at least some of the problems related to frequent mode transitions of wireless communication devices in a heterogeneous network.